In electrical and mechanical systems, it is often desirable to know whether a switch--typically an electrical switch located somewhere within the system, usually at a customer location--is in an open or closed condition. By the term "switch" is meant any electrical or mechanical means for obtaining a switching function. A "mechanical switch" includes any physical object which can be moved into a predetermined position. An "electrical switch" includes any means forming an electrical path which can be altered (switched) from or to a short circuit (ON) to or from an open circuit (OFF). Electrical switches include electronic and electro-mechanical types of switches. Examples include not only a conventional electrical switch, such as an electronic transistor or a mechanical wall switch, but also, for example, a wire which can be broken or cut and an electrical path in electrically conducting equipment or machinery which, due to mechanical movement, alternately opens and closes.
It is desirable to be able to determine (or interrogate), typically at a remote central office, the open vs. closed (OFF vs. ON) condition of the switch. As complete optical communication systems are developed in the art and put into commercial use, electrical lines (links) between the central office and any other part of the system, such as the customer location, may not be available for such interrogation, and it would be expensive to provide such electrical lines. The optical fiber links in these optical systems, as well known, have many advantages over electrical lines, such as strength and small size, and are particularly advantageous in certain corrosive or explosive environments where electrical wires are hazardous. Moreover, in an environment of electrical noise caused by switching equipment, motors, and so forth, where electrical interference can be a problem, it is desirable to have a nonelectrical link from a central location to a remote location for switch interrogation connections.